Fiber optic acoustic sensor arrays have been in development for years as a replacement for electronic-based (typically piezoelectric) sensor arrays. An exemplary use for such arrays is in connection with underwater sonar applications. A driver for this change in technology has been the fact that fiber optic sensors eliminate the need for electronics in the wet end of the system (i.e., in the water). The design and packaging of electronics to survive in a seawater environment is complicated, and has been a major cost contributor to underwater sonar systems. In addition, the resulting reliability of electronics in these systems has been less than optimal.
A generally accepted method of making fiber optic hydrophones for such sonar applications has been the air-backed mandrel (i.e., winding/bonding optical fiber around the outside of a flexible hollow cylinder). As the cylinder responds to acoustic pressure waves (e.g., underwater acoustic signals), the wound fiber varies in length, which causes a phase shift in the light passing through the optical fiber. The phase shift is measurable when the sensor is configured as an interferometer.
It is known that applying certain coatings directly to the entire optical fiber during manufacturing (and prior to incorporation into a sensor) can increase the acoustic sensitivity of the optical fiber (e.g., by a factor of 100 or more). This development may substantially eliminate the need for an air-backed mandrel to enhance the acoustic sensitivity of the optical fiber; however, the process to apply such a coating directly onto the entire optical fiber (as well as a methodology to package the coated fiber into an acoustic array) is labor intensive and therefore, often cost prohibitive.
Thus, a need exists for, and it would be desirable to provide, improved fiber optic acoustic sensor arrays and systems, and methods of fabricating the same.